The invention relates to making parts comprising a metal substrate provided with a protective coating forming a thermal barrier.
The field of application of the invention is making parts capable of retaining their mechanical properties at high temperatures, in particular gas turbine parts such as turbine blades, in particular for turbojet engines.
To improve the performance of gas turbines, and in particular the efficiency thereof, it is desirable to run them at temperatures that are as high as possible. To make parts for the hot portions, it is well known to use superalloys. As a general rule, superalloys comprise nickel as the main component and additional elements usually selected from chromium, cobalt, aluminum, molybdenum, titanium, tantalum, and many others.
Operating temperature can be further increased by providing the metal substrate of such parts with a protective coating that constitutes a thermal barrier.
It is known for this purpose to make a multilayer protective coating comprising an outer layer of ceramic and a metallic bonding underlayer, in particular an underlayer comprising aluminum or some other metal such as platinum.
The bonding underlayer interposed between the superalloy metal substrate and the ceramic outer layer serves to perform the following functions:
enabling an alumina film to be formed and to persist on its surface, which film has xe2x80x9cadhesivexe2x80x9d properties to enhance retention of the outer ceramic layer;
to protect the substrate from corrosion due to oxidation by the oxygen in any ambient medium that has managed to pass through the outer ceramic layer; and
to constitute a diffusion barrier against elements of the metal substrate which could contaminate the alumina film, thereby spoiling the interface between the bonding underlayer and the outer ceramic layer, and thus spoiling adhesion thereat.
Including reactive elements such as yttrium, cerium, hafnium, or the lanthanides within the bonding underlayer reinforces the diffusion-barrier function and enhances persistence of the xe2x80x9cadhesivexe2x80x9d alumina film.
It is well known to form a bonding underlayer of the MCrAlY type (where M is a metal such as Fe, Ni, or Co) by using a physical vapor deposition method, e.g. by plasma sputtering, without giving rise to reaction with the substrate, adhesion between the bonding underlayer and the substrate being of a mechanical nature. By way of example, reference can be made to documents U.S. Pat. No. 4,055,705 and U.S. Pat. No. 5,824,423. Nevertheless, in order to obtain a thermally-stable underlayer, it is necessary for it to be relatively thick, typically not less than 50 micrometers (xcexcm) to 100 xcexcm, and that gives rise to a weight penalty.
Other known methods consist in making the bonding underlayer out of an intermetallic compound which can be of smaller thickness due to its thermal stability. An intermetallic compound comprising aluminum and platinum has been found to have good properties.
Thus, U.S. Pat. No. 5,716,720 describes a method consisting in forming a platinum layer electrolytically on a nickel-based superalloy substrate, and subsequently in performing vapor aluminization at a temperature higher than 1000 degrees Celsius (C.). Nickel from the substrate diffuses within the bonding underlayer. An alumina film is formed by heat treatment on the surface of the bonding underlayer prior to forming a ceramic outer layer, e.g. out of ytrried zirconia obtained by physical vapor deposition. A reactive element can be included in the bonding underlayer during the step of vapor aluminization.
U.S. Pat. No. 5,238,752 describes another method which consists in forming a bonding underlayer on a superalloy substrate in which the underlayer comprises an intermetallic compound, in particular a compound of aluminum and platinum. The bonding underlayer is made by pack cementation at a temperature higher than 985xc2x0 C. and it has a thickness greater than 25 xcexcm. An alumina film is formed by oxidation on the surface of the bonding underlayer prior to forming the ceramic outer layer, e.g. of yttried zirconia by physical vapor deposition.
Patent application EP 0 985 744 describes yet another method comprising forming a layer of platinum on a nickel-based superalloy substrate by electrodeposition or by chemical vapor deposition and depositing an aluminum layer which is made from a gaseous halide and which diffuses into the platinum layer. Desulfurization and surface descaling is performed after each deposition operation by heat treatment at a temperature higher than 1050xc2x0 C. in order to eliminate sulfur which is harmful to adhesion of the alumina film that develops on the surface of the resulting bonding underlayer. At that temperature higher than 1050xc2x0 C., it is inevitable that elements in the substrate will diffuse into the bonding underlayer.
A method of forming a bonding underlayer comprising platinum and aluminum is also described in patent U.S. Pat. No. 5,856,027. A platinum layer is formed on the superalloy substrate by electrodeposition prior to depositing aluminum by chemical vapor deposition, optionally together with reactive elements. The resulting underlayer presents an internal diffusion zone in which nickel diffused from the substrate is present.
With those known methods, the way in which the bonding substrate is formed gives rise to interaction with the substrate. The Applicant has observed that various elements of the superalloy substrate diffusing into the bonding underlayer can form undesirable precipitates therein that are liable in particular to affect the alumina film developed on the surface of the bonding underlayer. In addition, it is difficult to control the precise composition of the bonding underlayer.
An object of the invention is to provide a method enabling a protective coating to be formed on a superalloy substrate to form a thermal barrier and including a bonding underlayer made of a chemically stable intermetallic compound comprising aluminum and at least one metal from the platinum group, the bonding underlayer being of controllable composition, being capable of having relatively small thickness, being made substantially without interaction with the substrate that could cause elements of the substrate to diffuse into the bonding underlayer, and being capable of forming on its surface a thin and persistent adhesive film of alumina suitable for bonding an outer ceramic layer.
According to the invention, this object is achieved by the bonding underlayer being formed by using physical vapor deposition to deposit a plurality of individual layers alternately of aluminum and of a metal from the platinum group, and by causing the metals of the resulting layers to react together exothermally.
The term xe2x80x9cplatinum groupxe2x80x9d is used herein to mean the group constituted by platinum, palladium, rhenium, ruthenium, osmium, and iridium.
In a physical vapor deposition method, the substrate is heated solely by coming into contact with the cloud of vapor containing the element that is to be deposited. The temperature of the substrate is thus relatively low, since in practice it does not exceed 700xc2x0 C. The substrate thus remains at a temperature which is well below that at which elements of the substrate are liable to diffuse into the deposit being formed.
It will also be observed that physical vapor deposition methods make it possible to control the thicknesses of the individual deposited layers which, in association with the absence or near absence of any element diffused from the substrate, makes it possible to form a bonding underlayer of precisely controlled composition that is determined by the ratio between the thicknesses of the deposited platinum and aluminum layers.
According to a feature of the method, once the individual layers have been deposited, heat treatment is performed in order to cause the intermetallic compound to be formed by causing the metals of the deposited layers to react together.
Preferably, the heat treatment is performed at a temperature of not more than 900xc2x0 C. in order to avoid causing elements to diffuse from the substrate.
Also preferably, the heat treatment is performed in a non-oxidizing atmosphere, e.g. in a vacuum or in an inert atmosphere.
According to another feature of the method, the total thickness of the bonding underlayer is less than 50 xcexcm, typically lying in the range 3 xcexcm to 30 xcexcm. This thickness is very significantly smaller than that of prior art underlayers.
The individual layers, at least those of aluminum, are of a thickness that is less than 2000 nanometers (nm), preferably no greater than 1500 nm, and which can be no greater than 200 nm.
Thus, the number of individual layers can vary from a few, typically at least three, to several tens, or indeed several hundreds.
For a given total thickness, when the number of individual layers is relatively small, the bonding underlayer retains a laminated appearance within the resulting coating, but without that having any significant effect on the adhesion thereof.
However, for the same total thickness, when the number of individual layers is relatively great, then the resulting intermetallic compound bonding layer presents a structure that is homogeneous.
The thickness of each individual platinum layer as deposited can remain constant throughout the deposition process, or it can be varied. The same applies to the thickness of the individual layers of aluminum as deposited. In particular, it is possible to deposit several series of aluminum and platinum layers that are individually of relatively small thickness, with these series of layers being spaced apart from one another by at least one layer of platinum plus one layer of aluminum that are individually of relatively great thickness so that after heat treatment the bonding underlayer presents the appearance of a succession of homogeneous phases that are separated from one another.
In addition, and possibly in combination with the above, it is possible to provide for a fixed or varying ratio between the thicknesses of the platinum layers and the aluminum layers as deposited throughout the deposition process so that the intermetallic compound as finally obtained has a composition that is constant or that varies through the thickness of the bonding underlayer.
The individual layers are formed by physical vapor deposition, e.g. by evaporation under electron bombardment or by evaporation under arcing with or without the assistance of a plasma, or indeed by cathode sputtering using at least a first target constituting a source of the metal from the platinum group and a second target constituting a source of aluminum.
According to an additional feature of the method, at least one reactive element selected from yttrium, zirconium, hafnium, and the lanthanides, for example, is deposited in addition to the aluminum and the metal from the platinum group in order to be included in the bonding underlayer. The reactive element can be codeposited with the aluminum and/or with the metal from the platinum group, e.g. by using an alloy as a source.
According to an additional feature of the method, at least one metal other than the aluminum and the platinum can be deposited to further improve thermal stability, e.g. a metal selected from Re, Ni, and Co. This additional metal can be deposited in separate layers or it can be codeposited with the optionally reactive element.
The invention also provides a gas turbine part of the kind that can be obtained by the above method, and more particularly it provides a gas turbine part comprising a superalloy metal substrate, a bonding underlayer formed on the substrate and made of an intermetallic compound comprising aluminum and a metal from the platinum group, an adhesive film of alumina formed on the surface of the bonding underlayer, and an outer coating of ceramic anchored on the alumina film, in which part the bonding underlayer has a thickness of less than 50 xcexcm, preferably less than 30 xcexcm.
According to a remarkable feature, the bonding underlayer is free from any elements diffused from the substrate. In other words, the presence of elements from the substrate cannot be detected using the normally available analysis techniques.